Science!: Ants and Monkeys

In Africa, there lives a genus of plant called the Acacia. Within the limbs of this plant live hordes of stinging ants. These ants are aggressive and attack anything that comes near its home.

Nigel Raine, a researcher from University of London in the UK has been studying the relationship between the acacia plant and the ants for quite some time. During his observations, he noticed that though the plant provides shelter and nourishment for the ants, the ants completely avoid the flowers that bloom on them. "Acacias...have very open flowers, but still, the ants don't seem to go on to them. We wanted to know why," states Raine.

The acacia gets a pretty sweet deal, housing entire armies of ants who will guard the plant against animals which would like to eat it. However, when ants and plants co-exist, the former often sup from the nectar of the flowers, which can put a damper on the pollination process. "Some plants [protect their flowers] structurally, with physical barriers to stop ants getting on to the flower, or sticky or slippery surfaces that the insects can't walk on." Says Raine. "Acacias don't have these barriers."

So, how do the acacias keep the ants from stealing the nectar? Apparently, through bribes. The acacia plant produces "extrafloral nectaries," which are small stores of nectar found on the stems. These distract the ants from the motherload of nectar in the flowers, while still giving them a taste of what they love. The acacia also produces "beltian bodies" on its leaves, which are nutritious and delicious structures that keep their ant guardians happy and healthy.

However, when these bribes fail to work, the acacia considers a more sinister approach. Flowers are designed to produce an attractive aroma that pollinating insects can't seem to resist. However, the acacia plant produces a small that actually repels the ants it houses. These repellents are very specific to the ants the acacia tree houses. "The chemicals don't repel bees, even though they are quite closely related to ants. And in some cases, the chemicals actually seem to attract the bees," states Raine.

Raine and his colleagues believe that the repellents mimic pheromones that the ants naturally produce. When the scent was placed into a syringe and puffed over a group of ants, they became agitated and aggressive.